Collaborative Research: Marginal instability and deep cycle turbulence in the equatorial oceans

概述：在它被发现30年后，赤道湍流的深层周期仍然无法解释。气候预测模型无法准确地表示它，导致海洋热吸收方面的错误。这个项目汇集了在这个问题上取得最新进展的两个研究小组，以寻求统一的理解。结合现有的观测和新的大涡模拟(LES)，他们将探索季节和ENSO变化范围内以及所有三个赤道海洋的深层循环物理。以前未被开发的边缘不稳定的深循环特性将与数值模拟一起被用来(1)记录过去25年的深循环的历史，(2)描述深循环的机制，以及(3)开发用于气候模式的改进的参数化。智力优势：赤道上太平洋混合的广泛变异性体现为在相同地点和相同季节采样的湍流日交会热通量，但与厄尔尼诺/南方涛动(ENSO)循环的不同阶段重合。1991年厄尔尼诺现象初期，洋流缓慢，热通量相对较弱。在2008年与拉尼娜同时发生的那次巡航中，洋流的能量要大得多。由此产生的热流强了一个数量级，热量被输送到100米深。尽管震级有极大的差异，但这些混合区域表现出一个惊人的共性：强烈的湍流与一个明显的层重合，在该层中，梯度理查森数(Ri)保持在四分之一的两倍之内。Ri在一定深度范围内1/4附近的持续聚集表示边缘不稳定状态。这种接近临界的状态是通过大范围强迫(减少Ri)和湍流(增加Ri)之间的平衡来维持的。深层循环混合对气候至关重要，因为它对纬向流系统和海洋表面温度都有影响。在这次观测数据分析和大涡模拟中，研究人员将探索深循环的物理，并特别关注边缘不稳定的性质。以边际不稳定为指标，他们将从现有数据中记录深周期的历史和纵向相关性。这将导致对这一现象的更深入的理解，并提高在大尺度模式中将其影响参数化的能力。更广泛的影响：将研究长期海表面温度变化的原因，结果将有助于ENSO和年代际气候预测。该项目还将促进新的合作，支持一名初级研究人员，并补充现有的NSF观测项目。将制作两个关于赤道海洋学的报告，并在高中和社区大学进行介绍。

Overview: Thirty years after its discovery, the deep cycle of equatorial turbulence remains unexplained. Climate forecast models are unable to represent it accurately, leading to errors in ocean heat uptake. This project brings together two research groups who have made recent progress on the problem to pursue a unified understanding. Combining existing observations and new large-eddy simulations (LES), they will explore deep cycle physics over the range of seasonal and ENSO variability and in all three equatorial oceans. The previously-unexploited deep cycle property of marginal instability will be used together with numerical simulations to (1) document the history of the deep cycle over the past 25 years, (2) describe the mechanics of the deep cycle, and (3) develop improved parameterizations for use in climate models.Intellectual merit: The broad variability of mixing in the upper equatorial Pacific is exemplified by the turbulent diapycnal heat flux sampled at the same location and in the same season, but coinciding with different phases of the El Niño/ Southern Oscillation (ENSO) cycle. Early in the 1991 El Niño, currents were slow and the heat flux was relatively weak. In the 2008 cruise, which coincided with La Niña, currents were much more energetic. The resulting heat flux was stronger by an order of magnitude, and heat was transported to 100 m depth. Despite the extreme difference in magnitude, these mixing regimes exhibited a striking commonality: strong turbulence coincided with a distinct layer in which the gradient Richardson number (Ri) remained within a factor of two of the value of a quarter. Persistent clustering of Ri near 1/4 over a range of depths signifies the state of marginal instability. This near-critical state is maintained in the mean by a balance between large-scale forcing (which reduces Ri) and turbulence (which increases it). Deep cycle mixing is crucial to the climate via its effects on both the zonal current system and the sea surface temperature. In this observational data analyses and large eddy simulations, the investigators will explore the physics of the deep cycle with particular attention to the property of marginal instability. Using marginal instability as a proxy, they will document the history and longitudinal dependence of the deep cycle from existing data. This will lead to both a deeper understanding of the phenomenon and an improved ability to parameterize its effects in large-scale models.Broader Impacts: The causes of long-term Sea Surface Temperature variations will be examined and the results will contribute to ENSO and interdecadal climate prediction. The project will also foster a new collaboration, support a junior researcher, and complement an existing NSF observational project. Two presentations on equatorial oceanography will be developed and presented at high schools and community colleges.